It is common for elastomeric polymers such as styrene-butadiene rubbers having a styrene content of from about 20 percent to about 35 percent to be produced in organic solvents such as hexane. These polymers can be terminated using a number of different compounds including silane containing compounds to yield silane end-capped polymers. This siloxane termination may also result in an increase in the Mooney viscosity of the treated polymer similar to the increase that occurs during tin coupling. However, upon subsequent desolventization of the siloxane-terminated polymer through the use of either steam or heated water, an even larger increase in Mooney viscosity often occurs during the hydrolysis of siloxane groups such as pendant --SiOR groups on the siloxane end groups thereby leading to coupling of the polymer via formation of Si--O--Si bonds between two siloxane end groups.
Thus, heretofore, a process utilizing steam or heated water in the desolventization of siloxane end-capped polymers containing hydrolyzable groups such as pendant --SiOR groups has always been accompanied by an increase in the Mooney viscosity of the polymers due to hydrolysis and subsequent coupling that occurs between the terminal siloxane groups.
In an attempt to overcome this hydrolysis and coupling problem, U.S. Pat. No. 5,659,056 set forth to treat the polymer prior to desolventization with a viscosity stabilizing agent soluble in the solvent used to prepare the polymer. In particular, this viscosity stabilizing agent was an acid such as C.sub.1 to C.sub.12 aliphatic and C.sub.6 to C.sub.12 cycloaliphatic and aromatic carboxylic acids including acetic acid, propionic acid, butyric acid, decanoic acid, cyclohexanoic acid, benzoic acid and the like. This viscosity stabilizing agent was used in suitable amounts to stabilize the viscosity, generally in the range of from 0.8 to 1.2 mole equivalents of viscosity stabilizing agent per mole equivalent of anionic initiator.
Notably, the viscosity stabilizing agents set forth in U.S. Pat. No. 5,659,056 do not react with the siloxane terminal end groups of the polymer, but rather serve to neutralize the by-product lithium compounds in admixture with the polymer. It is in this manner that the stabilizing agents of U.S. Pat. No. 5,659,056 serve to stabilize the Mooney viscosity of a siloxane terminated polymer having at least one hydrolyzable substituent on the siloxane end group against substantial increases during contact with water. Moreover, it is acknowledged that an increase in Mooney viscosity can be slowed substantially using the method set forth in U.S. Pat. No. 5,659,056 not only during desolventization, but also during subsequent storage for a limited period of time where the polymer may be subjected to hydrolysis in the form of moisture in the air or in some other manner.
Unfortunately, however, it has been found that the process set forth in U.S. Pat. No. 5,659,056, does not actually prevent an increase in Mooney viscosity, but only slows substantially the rate of the hydrolysis reaction and, therefore, the rate of coupling of the polymer. Over a period of time, the slow hydrolysis of the siloxane end group will occur, thereby continuing the problem of increased Mooney viscosity and coupling of the siloxane terminated polymers.
Thus, while attempts have been successfully made heretofore to slow down the rate of the hydrolysis reaction which results in the coupling of the siloxane end groups of the polymers, the art has not provided a means or method by which to stabilize the polymer upon aging and essentially stop the coupling of the siloxane terminated polymers.